Flat UI Free - Design Framework (html/css3/less/js). Flat UI is based on Bootstrap, a comfortable, responsive, and functional framework that simplifies the development of websites.
http://designmodo.com/flat-free/

Wirefy is a browser based responsive wireframe tool, which allows you to create functional wireframes using standard elements. Think of it as the foundation for your content to be built on. This allows you to focus on the content rather than the design to get faster sign off. Wirefy is not a one-solution answer.
http://getwirefy.com

A curated project library for Hammer For Mac. Rock Hammer contains baseline typography, plus styling for common HTML elements including images, forms and tables, as well as navigation, responsive modules and widgets
https://github.com/malarkey/Rock-Hammer

A @grayghostvisuals and @zakkain Joint™ Typeplate is a “Typographic Starter Kit.” We don’t make aesthetic design choices, but define proper markup with extensible styling for common typographic patterns. A stripped-down Sass library concerned with the appropriate technical implementation of design patterns—not how they look.
http://typeplate.com

A lightweight (2kb) micro-library, elegant & minimalistic CSS3 grid system, made with only three main classes and 12 columns. It`s very easy to use and understand, pre-packed with some extra css helpers for mobile devices.
www.responsiveboilerplate.com